Vitreous fluorophotometry has been performed in patients with diabetes mellitus without retinopathy, patients with diabetes mellitus with nonproliferative retinopathy, and normal volunteer subjects, age- and sex-matched to the patients. A new method for evaluating blood retinal barrier permeability to fluorescein and diffusivity of fluorescein in the vitreous has been developed. The amount of fluorescein leakage into the vitreous of patients has been compared to that of the normal subjects. Correlations with other features of diabetes, such as the quality of diabetic control, the existence of subclinical neuropathy and nephropathy, and other complications were sought.